1. Field of the Invention
The present invention relates to a printed circuit board, and more particularly to a printed circuit board having high signal transmission quality.
2. Description of Related Art
Printed circuit boards (PCBs) typically consist of three principal components: a base, resin, and copper foil. The base for a PCB is often constructed from woven glass fiber cloth. In the process for manufacturing the PCB, the base is treated by passing it through a dip pan containing resin. The treated base is then passed through a set of squeeze rollers, and then a drying oven to cure. Then, the base is cut into a sized base which has a size of the PCB. Once the base has been prepared, copper foil is applied to one or two sides of the base, typically by electrodeposition. The copper foil is etched to form different signal traces on the PCB.
Referring to FIGS. 1 and 2, a PCB 1 includes a base 10 which includes a set of first fibers 11 and a set of second fibers 12. The PCB 1 defines an X-axis extending in a horizontal direction, and a Y-axis perpendicular to the X-axis. The set of first fibers 11 of the base 10 disposed in the X-axis direction are interlaced with the set of second fibers 12 of the base 10 disposed in the Y-axis direction. Resin is impregnated in gaps 13 among the fibers 11 and 12. Generally, each straight line segment of a signal trace 17 is arranged on the PCB 1 at an angle of 0, 45, 90, −45 degrees relative to the X-axis. In a PCB, some line segments of the signal traces 17 will cross the fibers 11 or 12 of the base 10 (such as the line segment of the signal traces 17 arranged at the angles of 45, −45 degrees of FIG. 1), and some line segments of signal traces will be coincident with the fibers 11 or 12 of base 10 (such as the line segment of signal traces arranged at the angles of 0, 90 degrees of FIG. 1). Because the fibers 11, 12 and the resin have different dielectric constants, impedances of different signal traces 17 vary over a large range according to the locations and the angles of the line segments of the signal traces 17 relative to the base 10. However, in a PCB, if the impedances of different signal traces vary greatly, the delay times of signals on different signal traces will be very different.